'622 THE INDUCTIONS OF BIOLOGY. 



thus arise gradually or all at once, in formal 'ons that are 

 continuous or discontinuous, are of two kinds. Faunas of 

 different eras, are distinguished partly by the absence from 

 one of types that are present in the other ; and partly lay the 

 unlikenesses between the types that are common to both. 

 Such distinctions between Faunas as are due to the appear- 

 ance or disappearance of types, are of secondary significance : 

 they possibly, or probably, do not imply anything more than 

 migrations or extinctions. The most significant distinctions 

 are those between successive groups of organisms of the same 

 type. And among such, as above said, the differences that 

 arise are, speaking generally, small and continuous where a 

 series of conformable strata gives proof of continued existence 

 of the type in the locality ; while they are comparatively 

 large and abrupt, where there is evidence that between the 

 deposit of the adjacent formations, a long period elapsed. 



Another general fact, referred to by Mr Darwin as one 

 which palaeontology has made tolerably certain, is that forms 

 and groups of forms which have once disappeared from the 

 Earth, do not reappear. Some few species and a good many 

 genera, have continued throughout the whole period geologi- 

 cally recorded. But omitting these as exceptional, it may be 

 said that each species after arising, spreading for an era, and 

 continuing abundant for an era, eventually declines and be- 

 comes extinct ; and that similarly, each genus during a longer 

 period increases in the number of its species, and during a 

 longer period dwindles and at last dies out. Having made 

 its exit, neither species nor genus ever re-enters. And the 

 like is true, even of those larger groups called orders. Four 

 types of reptiles that were once abundant, have not been 

 found in modern formations, and do not at present exist. 

 Though nothing less than an exhaustive examination of all 

 strata, can prove conclusively that a special or general form of 

 organization when once lost is never reproduced ; yet so 

 many facts point to this inference, that its truth can scarcely 

 be doubted. 



